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Chapter 19 Heat and the First Law of Thermodynamics 19-1 Heat as Energy Transfer 19-2 Internal Energy 19-3 Specific Heat 19-4 Calorimetry 19-5 Latent Heat 19-6 The First Law of Thermodynamics 19-7 The First Law of Thermodynamics Applied; Calculating the Work Problem 15 15.(II) When a 290-g piece of iron at 180°C is placed in a 95-g aluminum calorimeter cup containing 250 g of glycerin at 10°C, the final temperature is observed to be 38°C. Estimate the specific heat of glycerin. 19-4 Calorimetry—Solving Problems Example 19-4: Unknown specific heat determined by calorimetry. An engineer wishes to determine the specific heat of a new metal alloy. A 0.150-kg sample of the alloy is heated to 540°C. It is then quickly placed in 0.400 kg of water at 10.0°C, which is contained in a 0.200-kg aluminum calorimeter cup. (We do not need to know the mass of the insulating jacket since we assume the air space between it and the cup insulates it well, so that its temperature does not change significantly.) The final temperature of the system is 30.5°C. Calculate the specific heat of the alloy. 19-4 Calorimetry—Solving Problems The instrument to the left is a calorimeter, which makes quantitative measurements of heat exchange. A sample is heated to a well-measured high temperature and plunged into the water, and the equilibrium temperature is measured. This gives the specific heat of the sample. 19-5 Latent Heat Energy is required for a material to change phase, even though its temperature is not changing. 19-5 Latent Heat The total heat required for a phase change depends on the total mass and the latent heat: 19-5 Latent Heat Heat of fusion, LF: heat required to change 1.0 kg of material from solid to liquid Heat of vaporization, LV: heat required to change 1.0 kg of material from liquid to vapor 19-5 Latent Heat The latent heat of vaporization is relevant for evaporation as well as boiling. The heat of vaporization of water rises slightly as the temperature decreases. On a molecular level, the heat added during a change of state does not increase the kinetic energy of individual molecules, but rather break the close bonds between them so the next phase can occur. 19-5 Latent Heat Example 19-6: Determining a latent heat. The specific heat of liquid mercury is 140 J/kg·°C. When 1.0 kg of solid mercury at its melting point of -39°C is placed in a 0.50-kg aluminum calorimeter filled with 1.2 kg of water at 20.0°C, the mercury melts and the final temperature of the combination is found to be 16.5°C. What is the heat of fusion of mercury in J/kg? Problem 20 20. (II) A 35-g ice cube at its melting point is dropped into an insulated container of liquid nitrogen. How much nitrogen evaporates if it is at its boiling point of 77 K and has a latent heat of vaporization of 200 kJ/kg? Assume for simplicity that the specific heat of ice is a constant and is equal to its value near its melting point. 19-5 Latent Heat Problem Solving: Calorimetry 1. Is the system isolated? Are all significant sources of energy transfer known or calculable? 2. Apply conservation of energy. 3. If no phase changes occur, the heat transferred will depend on the mass, specific heat, and temperature change. (continued) 19-5 Latent Heat 4. If there are, or may be, phase changes, terms that depend on the mass and the latent heat may also be present. Determine or estimate what phase the final system will be in. 5. Make sure that each term is in the right place and that all the temperature changes are positive. 6. There is only one final temperature when the system reaches equilibrium. 7. Solve. 19-6 The First Law of Thermodynamics The change in internal energy of a closed system will be equal to the energy added to the system minus the work done by the system on its surroundings. This is the law of conservation of energy, written in a form useful to systems involving heat transfer. 19-6 The First Law of Thermodynamics The first law can be extended to include changes in mechanical energy—kinetic energy and potential energy: Example 19-8: Kinetic energy transformed to thermal energy. A 3.0-g bullet traveling at a speed of 400 m/s enters a tree and exits the other side with a speed of 200 m/s. Where did the bullet’s lost kinetic energy go, and what was the energy transferred? 19-6 The First Law of Thermodynamics Example 19-7: Using the first law. 2500 J of heat is added to a system, and 1800 J of work is done on the system. What is the change in internal energy of the system? 19-7 The First Law of Thermodynamics Applied; Calculating the Work The following is a simple summary of the various thermodynamic processes. Problem 34 34.(II) In an engine, an almost ideal gas is compressed adiabatically to half its volume. In doing so, 2850 J of work is done on the gas. (a) How much heat flows into or out of the gas? (b) What is the change in internal energy of the gas? (c) Does its temperature rise or fall?